The present invention generally relates to solid-state mass storage media and their operation. More particularly, the present invention relates to flash-based memory devices that comprise multiple volumes and adapted to operate by exposing volumes with different service levels for different types of data.
Non-volatile solid-state memory technologies used with computers and other processing apparatuses (host systems) are currently largely focused on NAND flash memory technologies, with other emerging non-volatile solid-state memory technologies including phase change memory (PCM), resistive random access memory (RRAM), magnetoresistive random access memory (MRAM), ferromagnetic random access memory (FRAM), organic memories, and nanotechnology-based storage media such as carbon nanofiber/nanotube-based substrates. These and other non-volatile solid-state memory technologies will be collectively referred to herein as solid-state mass storage media. Mainly for cost reasons, at present the most common solid-state memory technology used in solid-state drives (SSDs) are NAND flash memory components, commonly referred to as flash-based memory devices, flash-based storage devices, flash-based media, or raw flash.
Similar to rotating media-based hard disk drives (HDDs), SSDs utilize a type of non-volatile memory media and therefore provide persistent data storage (persistency) without application of power. In comparison to HDDs, SSDs can service a READ command in a quasi-immediate operation, yielding much higher performance especially in the case of small random access read commands. This is largely due to the fact that flash-based storage devices (as well as other non-volatile solid-state mass storage media) used in SSDs are purely electronic devices that do not contain any moving parts. In addition, multi-channel architectures of modern NAND flash-based SSDs result in sequential data transfers saturating most host interfaces. A specialized case is the integration of an SSD into a hard disk drive (HDD) to form what is typically referred to as a hybrid drive. However, even in the case of a hybrid drive, the integrated SSD is functionally equivalent to a stand-alone SSD.
Another difference between HDDs and flash-based SSDs relates to the write endurance of flash-based media. Briefly, flash-based memory components store information in an array of floating-gate transistors, referred to as cells. NAND flash memory cells are organized in what are commonly referred to as pages, which in turn are organized in predetermined sections of the component referred to as memory blocks (or sectors). Each cell of a NAND flash memory component has a top gate (TG) and a floating gate (FG), the latter being sandwiched between the top gate and the channel of the cell. The floating gate is separated from the channel by an oxide layer, often referred to as the tunnel oxide. Data are stored in a NAND flash memory cell in the form of a charge on the floating gate which, in turn, defines the channel properties of the NAND flash memory cell by either augmenting or opposing the charge of the top gate. This charge on the floating gate is achieved by applying a programming voltage to the top gate. The process of programming (writing 0's to) a NAND cell requires injection of electrons into the floating gate by quantum mechanical tunneling, whereas the process of erasing (writing 1's to) a NAND cell requires applying an erase voltage to the device substrate, which then pulls electrons from the floating gate. Programming and erasing NAND flash memory cells is an extremely harsh process utilizing strong electrical fields to move electrons through the oxide layer. After multiple writes to a flash memory cell, it will inadvertently suffer from write endurance problems caused by the breakdown of the oxide layer. With smaller process geometries becoming more prevalent, write endurance problems are becoming increasingly important.
Another difference between HDDs and NAND flash memory technology relates to data retention, that is, the maximum time after which data is written that the information is still guaranteed to be valid and correct. Whereas HDDs retain data for a practically unlimited period of time, NAND flash memory cells are subjected to leakage currents that cause the programming charge to dissipate and hence result in data loss. Retention time for NAND flash memory may vary between different levels of reliability, for example, about five years in an enterprise environment to about one to three years in consumer products. Retention problems are also becoming increasingly important with smaller process geometries.
Data access and reliability-related characteristics and requirements associated with volatile and non-volatile memory components are collectively referred to herein as service levels and encompass such requirements as persistence, validity, write endurance, retention, etc. In view of the above, to be considered as viable storage alternatives to HDDs, SSDs using flash-based solid-state mass storage devices are required to meet certain service levels that include write endurance and retention time. Write endurance can be addressed by, for example, wear leveling techniques based on the number of P/E (program/erase) cycles among memory blocks. The retention constraint has mandated various mechanisms. As an example, the number of P/E cycles may be limited to satisfy the service level probability of the retention requirement. Strong error correction, such as through the use of error checking and correction (ECC) algorithms, can also be applied to reduce errors over time. With decreasing process geometries, constant data scrubbing is required to counteract increasing failure rates associated with retention. As known in the art, scrubbing generally refers to refreshing data by reading data from a memory component, correcting any errors, then writing the data back, usually to a different physical location within the memory component.
Flash-based memory technologies are seldom used as a system memory replacement in host systems, as opposed to a mass storage replacement that takes advantage of the large capacity of flash-based memory components. An intuitive example is in the use of swap files. Modern computer system memory are typically made up of random access memory (RAM) integrated circuit (IC) components, often SDRAM (synchronous dynamic random access memory). As the RAM area of system memory may often be limited and insufficient, operating systems of computers often use the disk area of an HDD as a swap file to temporarily dump and retrieve memory. Another example of such usage is by applications, such as temporary space by databases (e.g., TempDB in SQL Server).
Although traditionally provided by HDDs, system memory replacement does not require the long retention or even persistency offered by HDDs. A typical retention period for a swap file is very short (typically a few minutes at most) and can be limited to a day. Hence, mechanisms that limit the P/E cycles can be relaxed for such files or data, as there is no requirement to be able to read the data very far into the future.
Other applications that have been introduced with flash-based media can further relax more constraints. For example, a flash read cache application can handle loss of data as it can use the production volume's data (the cache just holds a local copy of the same data). That is, the application can use error detection mechanisms to verify data correctness and can tolerate data errors returned from the flash-based media. Also, persistence is not required in this case as again, the reference copy of the data is always available. Hence, such applications can allow even lower levels of data assurance.
In view of the above, different applications require different service levels from flash-based media in terms of retention, persistence and validity, and the use of a device with the highest service level for all data places unnecessary constraints and reduces efficient utilization of flash-based media.
The concept of exposing different logical unit numbers (LUNs), representing multiple volumes, with different service levels is well known in the storage industry. For example, U.S. Patent Application Publication No. 2010/0169570 addresses the issue from a performance perspective, providing different quality of service (QoS) levels to each volume. That is, volumes are configured to provide different performance metrics (for example, input/output (IO) operations per second (IOPS), bandwidth, latency, etc.) and assigned to different applications according to their importance and requirements.
However, flash-based storage is by nature a high performance volume. Hence, the above service level characteristics do not apply for such volumes. Instead, and as mentioned above, service level characteristics for flash-based storage generally relate to the endurance and retention levels of the data. These characteristics have been addressed from different perspectives. For example, U.S. Pat. No. 8,621,328 discloses memory that is logically divided into regions, and in which data are stored applying different error correction for dynamic data and static data.